Induction cook tops are typically equipped with multiple induction coils that define respective cooking zones. These coils are driven by high frequency currents to produce a magnetic field that is picked up by the ferromagnetic cooking utensil (e.g., pot or pan). The induced eddy currents in the utensil cause the utensil to heat up. The power delivered to the utensil to control the heat-up rate and capacity is varied by adjusting the operating parameters of the induction coil, particularly the converter frequency and/or operating voltage.
The induction coils are typically driven at a high frequency (e.g., around 20K-50K Hz range) that is above the threshold of human hearing. An issue arises, however, when multiple coils are operated simultaneously at different frequencies. Intermodulation of the driven frequencies results in a frequency that is essentially the difference of the driven frequencies (or harmonics thereof), and which may lie in the human audible range. This noise can be an irritant to certain consumers.
Various efforts have been proposed to eliminate or suppress the induction coil noise in induction cook tops. For example, U.S. Pat. No. 7,504,607 proposes to alter the operating frequencies of multiple induction coils so that the resulting superposition frequency is either below a first cut-off frequency or above a second cut-off frequency, with the cut-off frequencies being below or above the audible threshold values. Reference is also made to U.S. Pub. No. 2001/0079591 and U.S. Pub. No. 2008/0087661.
The prior proposed solutions seek to eliminate the induction noise from simultaneously operated coils by manipulating the power and frequency characteristics of the devices to suppress the noise altogether or render the noise inaudible to humans. The present invention seeks to address the problem in a fundamentally different and novel manner.